InvalidParameterException: English is not a valid Argument!

We are facing one issue in which whenever we try to access any WebDynPro application (My Expense or My TIme or Workflow Inbox) getting the error message

InvalidParameterException: English is not a valid Argument! and sometimes

IllegalArgumentException: English is not a valid Argument!

This is affecting a single user (VIP user).

This is ver very critical.

Please find the logs below:

#1.5 #00096B6E8BFC00270000020C001210B0000483A9220C23D6#1270662350774#com.sap.tc.webdynpro.clientimpl.http.client.ClientInfoObject#sap.com/tcwddispwda#com.sap.tc.webdynpro.clientimpl.http.client.ClientInfoObject#gopakr#290##n/a##66044ab0426d11df989d00096b6e8bfc#SAPEngine_Application_Thread[impl:3]_10ID\#(J2EE1268000)ID0809234950DB10469898144471064478End.WID1270651860779##0#0#Error#1#/System/UserInterface#Java###Failed to create locale for language: english

[EXCEPTION]

#1#java.security.InvalidParameterException: english is not a valid argument!

at com.sap.tc.webdynpro.clientimpl.http.client.ClientInfoObject.getLocaleFromString(ClientInfoObject.java:177)